1. Technical Field
The present invention relates generally to noise suppression devices and systems and, in particular, to in-line modules for attenuating electromagnetic interference transmitted to or from electrical and electronic circuits.
2. Description of the Related Art
Electronic systems such as power supplies and computers often radiate and conduct electromagnetic noise at various frequencies. Such emissions cause undesirable electromagnetic interference (EMI) that negatively impacts the host circuitry as well other electronic equipment in spatial or electrical proximity to the system.
The dramatic proliferation of electronic processing devices, such as personal computers (PCs), has resulted in printed circuit board (PCB) designers being faced with increasingly strict electromagnetic compatibility (EMC) regulations. For example, the Federal Communications Commission (FCC) has enacted the so-called “open chassis” EMI regulations that strictly limit the electromagnetic emissions that subject devices may generate. The open chassis criteria generally transfer responsibility to the manufacturer for the regulation of radiation generated by on-board components, such as integrated circuits (IC) modules, from a shielded enclosure (e.g., a computer chassis or enclosure) housing the components to the housed components themselves, such as a PC motherboard. Such emissions, which would otherwise be sufficiently attenuated by the shielded enclosure to satisfy the prior “closed chassis” regulations, must now be sufficiently attenuated independently of enclosure shielding. Satisfying the open chassis regulations enables radiating devices, such as motherboards, to be more flexibly distributed independent of a particular shielded enclosure.
The FCC's open chassis regulations are specified in Table 1 below.
TABLE 1Field Strength Limits for UnintentionalRadiators at a Distance of 3 MetersFrequency of EmissionField Strength(MHz)(microvolts/meter)30–88100 88–216150216–960200>960500
Given the tendency of EMI to generate undesired high-frequency current loops in the motherboard of a computing system and in view of the foregoing regulations, EMI is a major limitation on increasing processing speed.
So-called common-mode current is the primary source of EMI in many electronic systems. Common-mode is a current conduction mode in which currents, present in two or more conductors, are flowing in phase and with equal magnitude within each conductor. Common-mode current is the component of signal current that induces electric and magnetic fields that do not tend to cancel each other. For example, in a circuit with one outgoing signal conductor and one return “ground” conductor, the common mode current is the component of total signal current that flows in the same direction in both conductors.
Efforts undertaken to address the growing problems and limitations associated with common-mode EMI in electronic systems include meticulous layout of traces in PCBs, decoupling capacitors, and multiple-layer PCBs. Each of these techniques suffers one or more drawbacks in terms of PCB board space economy, high frequency limitations, and cost and complexity. While chassis shielding has long served as a primary EMI attenuation technique, and as explained above, current FCC regulations in combination with the desire for manufacturing flexibility, necessitate additional EMI control independent of that provided by chassis shielding.
In addition to the foregoing shielding and decoupling techniques, common-mode EMI may be suppressed using a ferrite device, often referred to a common-mode choke. Ferrites are utilized as an inexpensive supplement or alternative to suppress common-mode EMI in the electronic industries. A ferrite is generally a type of ceramic material often comprising divalent metals having unique molecular properties that generate high magnetic flux density in the presence of a magnetic field. Ferrites are typically axially mounted around signal path media such as pins or wires. Ferrite beads, cores, or other dissipating elements function by selectively absorbing electromagnetic energy at EMI frequencies. More specifically, a ferrite device absorbs reactive power in its magnetic field, which induces a voltage that opposes the interference signal current flow (typically single direction common mode interference current). Simply put, the common-mode interference signal is trapped in the electrical and magnetic fields of the ferrite and is dissipated as heat.
While effective for suppressing EMI in a variety of electronic systems, there remain problems with conventional design and deployment of such devices. One problem relates to the need for deploying such a device, such as a ferrite bead or toroid, onto a wire that is pre-connected at its ends. To address this problem, split or bisected ferrite devices are commonly utilized to permit installation on pre-connected cables. However, given the very tight geometric tolerances affecting ferrite performance, bisecting the ferrites increases the likelihood of slight geometric anomalies that may adversely affect EMI suppression performance.
Another significant problem associated with conventional ferrite design and deployment relates to the relative lack of tunability for a given application. A common expression of a ferrite's performance is in terms of permeability (μ). Permeability is the ratio of the magnitude of magnetic induction to magnetizing force and is variable as a function of frequency. The performance of a ferrite may be customized by adding metal oxides in various amounts to adjust the permeability and impart attenuation characteristics suited for a specified EMI suppression application. Therefore, setting of ferrite performance for a particular application requires relatively precise pre-estimation of the expected EMI environment. Given the dynamic nature of electronic system environments in which varying operating conditions and system wear may result in significant variations in the EMI environment, and in which environmental conditions such as temperature affect ferrite performance, conventional ferrites are often not optimally tuned for a given environment.
It can therefore be appreciated that a need exists for an improved device, system and method that addresses the foregoing problems. The present invention addresses these as well as other problems unaddressed by the prior art.